Fix for Issue#319

Concerning #319 

I added a interpretDouble() method to the Formula class and changed all interpretFloat() method call's which where used to represent the Formula value to interpretDouble, because I suppose that the conversion from double to float was producing the precision failure.

Jenkins Testrun:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/1023/